Lead-acid storage batteries comprise several cell elements each encased in separate compartments of a container containing sulfuric acid electrolyte. Each cell element comprises at least one positive electrode, at least one negative electrode, a porous separator (i.e., a thin microporous sheet and/or absorbent glass mat) therebetween. The electrodes each comprise a reticulated lead (including lead alloys) substrate, called a grid, which supports an electrochemically active material thereon, and conducts electrical current throughout the electrode. The active material comprises a leady material (i.e., PbO, PbO.sub.2, Pb or PbSO.sub.4 at different charge/discharge stages of the battery) pasted onto the grid.
Lead alloys are commonly used as a grid material to provide such properties as stiffness, strength, grain refinement, hardness, corrosion resistance, processability and conductivity. Increased strength and hardness, for example, are attributable to the presence of alloying atoms in the lead that interfere with the movement of dislocations in the crystals. The alloy contains many crystals or grains that are in various orientations, and are defined by grain boundaries formed during solidification of the alloy. For example, silver atoms in a lead alloy introduce non-uniformities within the crystal lattice which block the movement of dislocations and form a network of silver-rich grain boundaries that pervade the grid and extend to the surface of the grid where they contact the active material.
Antimony-free alloys, such as lead-calcium-tin alloys, and lead-calcium-tin-silver alloys, are known for use in so-called "maintenance-free" lead-acid batteries. Elimination of the antimony prevents electrolytic decomposition of the electrolyte and consequent gassing of the battery. Gassing results in a loss of electrolyte from the battery, and requires periodic additions of water over the life of the battery. Pb--Ca--Sn--Ag alloys have been used for positive grids in maintenance-free batteries and may comprise, by weight, about 0.025% to about 0.2% Ca, about 0.1% to about 2.5% tin, about 0.015% to about 1.5% silver, and may contain a some (e.g., about 0.005-0.01%) aluminum to prevent calcium loss during melting. The alloy may be (1) cast directly into grids, (2) cast into strips which are subsequently punched or expanded directly into grids, or (3) cast into ribbons (e.g., 3.5 in wide.times.5/8 to 2 in. thick) which are rolled in a rolling mill to form wrought alloy strips (e.g., 3.5 in. wide.times.0.034-0.050 in. thick) which are subsequently punched or expanded into grids.
Cast lead alloy grids are well known in the art and provide a microstructure characterized by a plurality of conjoined irregular polyhedral grains that meet one another at a grain boundary. The grain dimensions range between about 0.025 mm to about 0.100 mm with a maximum length to height aspect ratios between 1.0 to 4.0. The major axes of the grains are not aligned due to any particular manufacturing process (e.g., rolling), and there will be about 200 to about 1000 grains per square millimeter on the grid surface contacting the active material. Certain cast alloy grids (e.g., lead-calcium-tin-silver) are known to promote good adhesion with the active material as well as provide a conductive oxide layer between the grid and the active material, apparently by providing a conductive network throughout the oxide that corresponds to the original grain boundary network of the cast metal. This good adhesion and conductive network is believed to contribute substantially to the extended high-temperature (i.e., 75.degree. C.) cycle life obtained with such grids, as determined by the SAE J240B test regimen.
Lead-acid batteries have also been made from wrought lead-calcium-tin alloys which are expanded to form the grid. Ribbons of the alloy are cast (e.g., drum cast or pulled from a pull box) and fed into a rolling mill for rolling into strips which are anywhere from about 20 to 60 times thinner than the thickness of the starting ribbon. The rolling produces flat and elongated grains that are longitudinally aligned in the direction of rolling. These elongated grains typically have a length of about 0.5 mm to about 5 mm, a width (i.e., in the plane of the strip) of about 0.025 mm to about 0.100 mm, and a thickness (i.e., in the direction of the thickness of the strip) of about 0.001 mm to about 0.005 mm. Lead-acid batteries having positive electrodes made from wrought lead-calcium-tin alloys typically have a longer SAE J240B ambient temperature (i.e. .ltoreq.40.degree. C.) cycle life than those made from grids cast from the same alloy. This is due to the excellent ambient temperature corrosion resistance of the wrought alloy which is characterized by grains which are large and flat (i.e., in the direction of rolling) with fewer inter-granular boundaries at the surface of the grid (see FIG. 1). Wrought grids, however, are relatively short-lived as determined by high-temperature (i.e., 75.degree. C.) life cycle test (i.e., SAE J240B) owing to corrosion of the grid surfaces which forms an electrically resistive layer (see FIG. 2) between the active materials and the grid, and seemingly reduces the electrical and physical connectivity between the active material and the grid. In this regard, it is believed that the rolling of the alloy in the rolling mill so reduces the number of silver-rich grain boundaries at the surface that when a corrosion layer forms it is more electrically insulating than if there were a higher concentration of grain boundaries at the surface as would occur in grids made from cast alloys of the same composition. In this regard, the rolling operation extends the surface of the strip by the same amount as its thickness is reduced (i.e., 20:1-60:1) which correspondingly decreases the number of grain boundaries that are present per unit surface area of the strip. Typically, a wrought strip will have only about 5 to about 100 grains per square millimeter, and a corresponding number of grain boundaries.